Field of the Invention
The present invention relates to a bobbin for winding optical fibers, such as an optical fiber for communication. Particularly, the present invention relates to a bobbin that is suitable for allowing an optical fiber to ride over a flange portion from an auxiliary winding drum and winding the optical fiber around a main winding portion as a bobbin having the auxiliary winding drum at the end portion, and an optical fiber wound around the bobbin.
Description of the Related Art
Bobbins in which flange portions are provided at both ends of a winding drum (a main winding drum to be described below) that forms a substantially cylindrical shape are used as bobbins for winding an optical fiber. Meanwhile, it is usual to perform various kinds of measurement, such as measurement of transmission loss, and tests for quality assurance after an optical fiber for communication is wound around a bobbin. Therefore, it is necessary to expose both end portions of the lengthy optical fiber in a state where the optical fiber is wound around the bobbin, for example over a length of about several meters to about several tens of meters.
Since a winding finish end portion is located at the outermost periphery on the winding drum of the bobbin, it is natural that the optical fiber be exposed. However, a winding start end portion is located at the innermost periphery of a winding layer in which the optical fiber on the winding drum is layered and is buried in the winding layer when the optical fiber is simply wound. Thus, generally, for example as shown in FIG. 16, a bobbin 10 in which an auxiliary winding drum (also referred to as a dead winding portion) 16 is provided outside at least one main flange portion 14A (a winding start side) out of main flange portions 14A and 14B of a winding drum (main winding drum) 12 is frequently used. In the winding of the optical fiber, after the winding start end portion of the optical fiber is wound around the auxiliary winding drum 16 over about several meters to about tens of meters, the optical fiber is guided between the pair of main flange portions 14A and 14B, and the optical fiber is wound on an outer peripheral surface of the main winding drum 12 between the pair of the flange portions 14A and 14B.
In the above case, when a simple circular flange shape is given to the main flange portion 14A for starting winding without exercising particular ingenuity, the optical fiber is allowed to ride over an outer peripheral edge of the main flange portion 14A in order to guide the optical fiber from the auxiliary winding drum 16 to the main winding drum 12. Moreover, the optical fiber is guided to the outer peripheral surface of the main winding drum 12 along an inner surface of the main flange portion 14A, and winding in the main winding drum 12 is started in that state. However, in this case, the winding start end portion of the optical fiber along the inner surface of the main flange portion 14A and the optical fiber wound on the outer periphery of the main winding drum 12 cross over and overlap each other. In this way, the winding start end portion where the optical fibers cross over and overlap each other is generally referred to as a crossover wire. In a position of such a crossover wire, lateral pressure is applied to the optical fiber, and bending occurs in the optical fiber in the portion where the lateral pressure is applied. Therefore, there is a negative influence on these transmission loss properties of the optical fiber. Thus, it is desirable to guide the optical fiber from the auxiliary winding drum 16 to the main winding drum 12 so that no crossover wire is generated.
As a technique for avoiding generation of the crossover wire, as shown in FIG. 16, a hole 18 passing through the main flange portion 14A is formed at the position on the main flange portion 14A for starting winding corresponding to the vicinity of the outer peripheral surface of the main winding drum 12. The winding start end portion of the optical fiber is inserted into the hole 18 from the inner surface of the main flange portion 14A, and is pulled out to an outer surface of the main flange portion 14A, the optical fiber is reeled over a certain degree of length via the hole 18 and wound around the auxiliary winding drum 16, and then the optical fiber is wound around the main winding drum 12 (for example, Japanese Unexamined Utility Model Application, First Publication No. H2-4866 and the like). However, this technique depends on human manual work of reeling a certain length of the optical fiber and winding the optical fiber around the auxiliary winding drum 16 after an optical fiber as thin as a hair or the like is passed through the narrow hole 18. Therefore, it is very difficult to mechanize or automate work at the start of winding. Therefore, full-automation of winding work is difficult, and there are also limitations to increasing efficiency of work, and cost reduction.
Thus, as shown in FIG. 17, a slit 17 which reaches the outer peripheral surface of the main winding drum 12 in a direction toward the center from the outer peripheral edge of the main flange portion 14A is formed in the main flange portion 14A that is the winding start end portion, and guiding the optical fiber from the auxiliary winding drum 16 through the slit 17 to the main winding drum 12 is widely performed (for example, Japanese Patent Publication No. 2806327, Japanese Patent Publication No. 3961991, or the like). According to such a technique, when the optical fiber is guided from the auxiliary winding drum 16 to the main winding drum 12, since the optical fiber just needs to be dropped into the slit 17 from the outer surface of the main flange portion 14A, it is possible to automate the work.
However, since the slit 17 is formed in the main flange portion 14A, the strength and rigidity of the main flange portion 14A deteriorate greatly. Therefore, when the optical fiber is wound or delivered while rotating the bobbin at a high speed, there is a concern that the main flange portion and its root portion may be damaged. Particularly, in recent years, since an optical fiber wound around a bobbin also becomes lengthy and a force applied to the main flange portion becomes large, there has been a high possibility of the main flange portion being damaged. Additionally, if the slit as described above is provided in the main flange portion, when the bobbin around which the optical fiber is already wound is gripped by hand or the like for transfer and the like, the main flange portion may warp due to insufficient rigidity and collapse (winding collapse) of an optical fiber winding layer may occur.
In addition, although the deterioration in the strength or rigidity of the main flange portion accompanying the formation of the slit can be compensated for by reinforcing the main flange portion with ribs and the like, there are also limitations to this compensation. Therefore, it is difficult to reliably prevent damage to the above main flange portion, and the weight of the bobbin increases or costs of materials also rise.